HELP! OAG at 2800mm focal length

[x6gas]

Calling all long focal length deep-sky imagers out there!

I've finally got around to trying some DSO imaging using my Celestron HD11. I was trying to capture M101 and had no trouble finding and framing the galaxy but had no luck at all finding a suitable guidestar.

I am using an Atik OAG with a QHY IMG0H as a guidecam which has a pretty sensitive Sony 618 chip...

I've come to the conclusion that Off-axis guiding at this focal length clearly needs some planning rather that the set-up and hope approach that has served me so well when imaging with my short focal length triplet. So can anyone give me some advice on how to plan a session? I think maybe it's time to discover the pleasures of CdC but how do you determine where the guide chip is relative to the imaging chip?

A pointer to an idot's guide would be very welcome.

Actually, any advice at all would be very welcome; I've just spent the whole evening hunting for a guide star and failing...

[Merlin66]

I use a C11 with a Vixen beamsplitter, similar to the OAG (but much better FOV!) at f10 with the spectroscope.

A couple of comments:

1. The OAG/ guide camera and the imaging camera MUST be brought to the same focus BEFORE you go under the stars.

2. If you initially orientate the OAG either along the RA or Dec axis then you know which way to move a star to bring it into the guide FOV.

3. You can certainly use the overlays in CdC to show the imaging FOV and the OAG size and relationship.

I use either the QHY5, or an ATiK16ic or a Lodestar for guiding - usually a 1 to 1.5s setting in PHD. Does your camera allow long exposures??

http://stargazerslounge.com/discussions-scopes-whole-setups/90916-orion-oag-first-light.html

This thread may be helpful.

If I can assist further just let me know.

[x6gas]

Thanks for the reply Merlin.

Should have said that the imaging and guide cameras have already been correctly focused using my 90mm frac set up so I don't think that's the problem.

I was using exposures of up to 10 seconds on the guidecam last night (using PHD) but was getting a very noisy display. I am actually now wondering if there was some kind of problem with the guide camera... It is certainly good to know that you can guide with exposures of a second or so using a C11. Do you generally have any problem finding a suitable guide star?

Any idea how to work out the relative positions of the imaging and guide sensors to plug in to CdC?

[NickK]

You can adjust the prism depth if you have a smaller sensor.

[Merlin66]

Never have a problem finding guide stars.

If the guide camera is sitting above the imaging say in Dec, just centre a bright star in the imaging FOV, and slowly, gently move the scope in Dec until the same star appears in the guide FOV...simples!

(You can use CdC at high magnification to verify the off-set)

Do this, and confirm the guide camera can indeed see and lock on to the target....

[ncjunk]

Never have a problem finding guide stars.

If the guide camera is sitting above the imaging say in Dec, just centre a bright star in the imaging FOV, and slowly, gently move the scope in Dec until the same star appears in the guide FOV...simples!

(You can use CdC at high magnification to verify the off-set)

Do this, and confirm the guide camera can indeed see and lock on to the target....

Hah! you never have a problem because you have taken the time to align the guide camera, check which way you have to move to put the star on the sensor and then find a suitable star and move to it. (Doing it correctly, in other words )

I, on the other hand, am basically a bone idle fool who keeps slewing to targets in the vain hope blind luck and fortune will bless me with the miracle of a guide star. I was imaging the C11 at F10 and there were a few targets that I had problems finding guide stars for. Some would have required rotating the camera and on some I used a very poor S/n ratio star.

So, if like Merlin you take the time to setup your system and configure the eyepieces in cdc so they show your imaging camera and guide camera you can then plan which guide star you can use and where you have to position the camera.

On the other hand if you have just been slewing to the target in the hope of a guide star, like me, then some targets will not have a suitable guide star.

With an ed80 the slew and hope method is wonderfull as theres always a guide star but at 2800 it´s not as successfull.

I also have had to buy some spacers (5mm and 10mm) in order to move the imaging CCD so that the Guide camera could achive focus (and the focus point was within the limited range of adjustability of the OAG)

I do keep meaning to set it up like Merlin honest!

I was using times of 2 - 4 seconds on the guide camera BTW. Obviously if you select a decent guide star then you can use the shorter times (depending on seeing etc).

[r3i]

Never have a problem finding guide stars.

If the guide camera is sitting above the imaging say in Dec, just centre a bright star in the imaging FOV, and slowly, gently move the scope in Dec until the same star appears in the guide FOV...simples!

(You can use CdC at high magnification to verify the off-set)

Do this, and confirm the guide camera can indeed see and lock on to the target....

This is great advice - I do this each session just to check all is OK with my setup, it only takes a minute or so. The only thing I would add is that if you do this with a Bahtinov Mask in place, you can see the spikes appear in the guide camera's FOV long before the actual star comes into view, which helps in getting the star into the FOV of the guide camera.

I've also managed to get a guide star each time with my 1600mm FL RC using a Meade DSI1 OSC & Orion OAG. Typical exposure time of a couple of seconds. Sometimes, particularly on objects away from the Milky Way, I've had to rotate the guide camera to find a suitably bright guide star.

I've also set up CdC with the FOVs for my image and guide camera but haven't actually used them for any forward planning of which guide star to use - so far I've managed to find guide stars so haven't needed to be that organised. I guess though that a FL of 2800mm is in quite a different league from my RC and locating guide stars will be more of a challenge.

[x6gas]

Thanks, all for your replies. I am buoyed that this is possible and really am thinking that there must have been some kind of probelm with my guidecam last night...

If the guide camera is sitting above the imaging say in Dec, just centre a bright star in the imaging FOV, and slowly, gently move the scope in Dec until the same star appears in the guide FOV...simples!

This is exactly what I tried to do last night... after spending ages rotating the imaging train bit by bit to see if I could find a suitably bright star...

I shall re-double my efforts, check and double-check everything and report back!

[kookoo_gr]

using an oag is a bit frustrating at the beginning but only until you manage to set it up and learn how to find a guide star. here is what i did with my rig and i always find a guide star even under LP skies.

1. Focus both cameras during day time (obviously you have done this step)

2. Under the night sky find a bright star e.g. Sirius, or any other bright star from the constellations. Slew your scope and center your imaging camera at it and write down the coordinates (RA/DEC) that your handset/EQMod shows.

3. Fire up PhD set it at 2 - 3secs (that's the setting at the time interval i am using when guiding) and capture some dark frames, it helps a lot to locate a star, for now that you will use a bright star for initial setup you can use 1 sec.

4. Rotate the OAG so that the guide camera is facing vertically down or horizontally to the left or right. (that way the guide camera is aligned to either one of the axes of the mount)

5. Using your mount controls (or via EQMod) slew the scope so that the star you see in your imaging camera is placed at the edge of the FoV.

6. Without moving any parts from the OAG, move that star along the edge of the imaging camera's FoV until you can see it at the guide camera, also when you find the star, try to place it as best as possible at the center of PhD's screen without touching the oag (use only the mount). then write down the coordinates where you found the star.

7. Subtract these coordinates and you have the difference between the two cameras and you can place a circle in CdC so you know how far away is the OAG in relevance to the imaging camera. As for the FoV of the guide camera you can calculate it the same way as you did for the main camera.

When doing step 6 if you can't find any star then move a bit more outside the FoV of the imaging camera and try again. If you can move the prism inward/outward then a good idea is to set the prism in the middle (if you move the prism at a later session then the fov you set up at CdC is no good) so that the stars will be as round as possible.

Once you have found the bright star use a bahtinov mask to compare the focus between the imaging camera and the guide camera and fine tune it. From the guide camera's point of view the bahtinov mask is almost useless so you will need to remove the mask and fiddle with the focus. Keep the focus perfect at the imaging camera and adjust the guide camera's focus. The best way to do it is to write down the difference in mm between the focus of both cameras and use the appropriate spacer. It is possible, when using the oag that the star(s) will not be round but flattened, remember that you use light form the edge of the mirrors so it is natural that the stars will be flattened. PhD has no problem using them for guiding as long as the focus is perfect and your scope's collimation is perfect.

Hope it helps

EDIT: the main thing you need to know is to always get dark frames for you guide camera, also check for bad connections at your guide camera and if necessary change the guide camera's usb cable

[x6gas]

Wow, thanks for the comprehensive advice which will be really useful.

The information in this thread convinced me that the issue was probably a problem with my guide cam and close inspection has revealed that one of the pins from the din lead supplying the camera has broken :-(

Murphy's Law that this should happen the first time I try guiding the HD11, but I'm kicking myself that I should have taken the guidecam inside and checked it out early in the process...

Now all I have to do is extricate the broken pin from the female socket and source a non-standard 9 pin din lead with an EMR suppressor.

Thanks again all - as I say, the info on this thread has been and will be really useful.

Cheers, Ian

[BlueAstra]

Recently I've been using my OAG with a 0.8FR in my Meade 5000 ED80. Absolutely no problem finding guide stars. When I had my C8 SCT I found it was slightly more difficult to find a good guide star with the 0.63FR, so I started using the ED80 setup in the C8 (ie with the 0.8FR). This increased the focal length of the C8 compared with the .63FR, but it was easier to find guide stars.